In 1987, a young urban folk singer named Tracy Chapman proved that populist lyrics against an acoustic backdrop can have tremendous mass appeal (it won her six Grammy nominations).

That same year, another folk singer, Suzanne Vega, hit the big time with the unlikely Top 5 single "Luka."That one-two shot of folk music had a lot of music experts predicting a rebirth of folk music. That talk, in turn, forced major record labels from Los Angeles to Nashville to London to re-evaluate how they treated folk musicians - traditionally regarded as the poor step-children of country music.

Chapman and Vega proved there was a ravenous market for it. But how exactly do record companies reach those millions and millions of thirtysomething, compact disc-buying, non-radio-listening, folk-rock fans searching for socially relevant statements that typified the 1960s?

Judging from what's happened since 1987, it's a question yet to be answered. And what many predicted would be a massive pop-folk music revival on par with the 1960s success of Bob Dylan, Joan Baez and Peter, Paul & Mary never really materialized. At least not on the commercial scale many had foreseen.

But what has become particularly evident over the past two years is the emergence of an entirely new blend of folk music (I prefer to call it "neo-folk") that shares a common heritage with traditional country, bluegrass, rhythm & blues, pop, Cajun, Tex-Mex, rockabilly and, especially, traditional folk.

It is primarily acoustic and primarily feminine, though it is virtually impossible to pigeonhole. It comprises a list of old and new artists as diverse as Bonnie Raitt (R&B) and Tish Hinojosa (Tex-Mex) and Nanci Griffith (folkabilly).

Some of it might get played on country stations, some even on Top 40. Most never gets played on radio stations at all.

Neo-folk is the kind of hybrid that folk stalwarts like Nanci Griffith and Christine Lavin have championed for years, though in relative obscurity. Only now they have been joined by an entire new generation, including Michelle Shocked and the Indigo Girls, who have found a profitable niche on the college radio charts.

A new crop of stars is on the horizon, and a particularly bright one is Shelby Lynne, whose "Tough All Over" (Epic) is being marketed as country music, but whose smoky, sexy voice is anything but.

She's not a traditional folk singer in that she writes her music or plays acoustic guitar. Her instrument is her voice, which, quite frankly, may be the purest to come along in years. And she can take material written by others and transform it into something magical.

She gives Duke Ellington's "Don't Get Around Much Anymore" a wistful turn, and her version of Johnny Cash's "I Walk the Line" will make you forget who sang it to begin with.

She can be tough ("What About the Love We Made"), she can be rowdy ("Lonely Weekends"), she can be bluesy ("Baby's Gone Blues") and she can be convincingly emotional ("I'll Lie Myself to Sleep"). There are 10 songs here, all of them sensational.

Another folk singer (this one also a songwriter) making a serious move toward mainstream acceptance is Mary-Chapin Carpenter, a veteran of the U.S. folk scene whose vastly underrated songwriting talents are now showcased on "Shooting Straight in the Dark" (Columbia).

She possesses one of those endearing voices that make you forget what kind of music she's singing and just sit back and enjoy it for what it is. Her music appeals to fans of all tastes, although last year's "State of the Heart" enjoyed particular success on the country charts.

As with Shelby Lynne, country music is an all-too-exclusive pigeonhole for Carpenter. Where Lynne is an extraordinary vocalist, Carpenter is an extraordinary songwriter in the mold of John Hiatt. Billboard Magazine referred to her as "one of the most powerfully poetic voices in music."

She comes not from Nashville, but from the Washington, D.C., folk circuit. She was educated at Brown University and records in Virginia - something that nevertheless didn't deter the Academy of Country Music from naming her "Top New Female Artist."

She has never eschewed country music, but neither has she labeled herself a country singer. She just doesn't like labels.

As she told one critic, "For many years I resisted being called a folk singer, more out of fear than anything else. Now I have to accept the fact that folk music, too, has this huge sort of embrace or umbrella, and people from Suzanne Vega to Shawn Colvin to the Indigo Girls to Cowboy Junkies, even, are all written about from the standpoint of folk musicians."

Now she lives with a country label that doesn't quite fit on "Shooting Straight in the Dark." Here, she employs folk singer John McCutcheon on the sensational "Halley Came to Jackson" (she returns the favor on McCutcheon's latest release) and Cajun stars Michael Doucet and Beausoleil on the delightful "Down at the Twist and Shout" - perhaps the finest of the 11 exceptional songs on the album.

The way her lyrical style interplays with a mix of folk, Cajun and rock makes "Shooting Straight in the Dark" one of the most remarkable releases of the year.

Now, with the lineage enjoyed by Rosanne Cash, you'd expect her music to be Carter Sister-country to the teeth. Guess again. Her latest album, "Interiors" (Columbia) is an acoustic-rock-flavored offering that falls just short of masterful.

The first actually written and produced by Rosanne Cash, "Interiors" is so starkly different from anything critics expected that reviews are mixed, at best. But taken in the context of the neo-folk movement, "Interiors" marks an emotional eclecticism that reaches straight for the heart.

From the punkish flirtation with substance abuse on "I Want a Cure" to the tragedy of child abuse on "This World," Cash is not afraid to address issues that are as home on country radio as they were with the Sex Pistols.

While she lacks the subtle poetry of Carpenter, the real strength of Cash's songwriting on "Interiors" is her ability to hit her subject matter - love, fear, anger - directly between the eyes. And that ability pushes her songs to new heights.

Of course, Rosanne Cash has always been somewhat of a wayward child of country music. Now "Interiors" pushes her further from that mainstream and into a neo-folk sphere - something that is likely to sell fewer records, even though the quality of her product is superior.

From her early associations with Gram Parsons, Emmylou Harris has been a virtual pioneer of female folk music, though lately her musical course has steered her more into the mainstream of country music. Now, with "Brand New Dance" (Reprise), she returns to those early 1970s folk roots - a sound that is surprisingly similar to the neo-folk of Lynne, Cash and Carpenter, among others.

In particular, there's a whole lot less country twang to the vocals, far fewer weeping pedal steel guitars and a whole lot more commercially appealing arrangements on "Brand New Dance."

Whether her recent country fans will buy her folk sound, and whether her old folk fans still remember how good she was in those pre-country days, will determine whether this album reestablishes her as a premier talent in American music or whether it will become a pleasant diversion enjoyed by only Emmylou purists.

The bottom line with all four releases is that each would appeal to a much wider audience than it is likely to receive. And each is a gem that defies musical labels.